Packing assemblies for sealing an annulus between the cylindrical surfaces of relatively movable members, such as a valve stem and a valve bonnet, commonly employ layers of packing rings which when subjected to axial compression are urged to expand radially into sealing engagement with the enclosing cylindrical surfaces. It is also a common practice to provide for two such sets of packing rings in tandem array separated by a lantern ring which is connectable through a pressure fitting in an accommodating port in the valve bonnet, to a fluid sensor or an external source of fluid lubricant or other injectable substance such as a sealing plastic to enhance sealing effectiveness. In assembly of valves, it is also a frequent practice to place the valve stem packings in a preloaded condition prior to use in order that a seal can be established at low fluid pressures as the pressure builds in the valve. Frusto-conical spring washers or belleville springs located at one end of a stack of packing rings are a conventional means for imparting an axial compression of the packing rings and a pre-loading of the packing structure. The springs also function to maintain an axial loading on the packing assembly should there be some loss or extrusion of the packing ring material.
It has been observed, however, that the loading placed on the packing rings by the belleville springs when located at an end of a stack of packing rings does not result in uniform sealing effectiveness for the several packing rings. The packing ring adjacent to and those nearest a belleville spring receive a greater axial and radial loading than those more remote from the belleville springs with a consequent reduction in the sealing effectiveness of the more remote rings and in the reliability and effectiveness of the packing assembly as a whole.